1. Field of the Invention
The present invention relates to a single-mode optical fiber used as a transmission line in optical communications or the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength division multiplexing (WDM) transmission.
2. Related Background Art
In recent years, there has been an increasing necessity to reduce nonlinear phenomena (distortions of signal light) which have become remarkable as long-haul transmission has been made possible with the advent of WDM transmission and optical amplifiers. Therefore, for example, Japanese Patent Application Laid-Open Nos. 8-304655 and 9-33744 have proposed dispersion-shifted optical fibers which reduce such nonlinear phenomena and are applicable to WDM transmission.
In these dispersion-shifted optical fibers, a core region surrounded by a cladding region has a ring-shaped core structure comprising an inner core and an outer core provided on the outer periphery of the inner core. The refractive index of the outer core is set higher than that of the inner core. Thus, these dispersion-shifted fibers are designed such that their dispersion slope becomes smaller and their effective area becomes larger, thereby enabling them to be suitable for WDM transmission.
Here, nonlinear optical effects refer to phenomena in which signal light pulses are distorted in proportion to density of light intensity or the like due to nonlinear phenomena such as four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and the like, thereby restricting transmission speed or repeater spacing in relay transmission systems.
Having studied the conventional dispersion-shifted optical fibers, the inventors have found the following problems. Namely, in the conventional dispersion-shifted optical fibers, in order to make the refractive index of the outer core become higher than that of the inner core surrounded thereby, GeO2 is added to the outer core in general. As the addition of GeO2, however, generates thermal expansion difference between the individual regions in each of optical fibers at the time of drawing the optical fibers, and further causes a drastic change of internal stress in each of the optical fibers, structural irregularity and glass defect are likely to occur in the vicinity of the outer core interfaces (including at least the interface between the inner and outer cores and the interface between the cladding region and the outer core). There is a possibility that the above-mentioned structural irregularity and glass defect become a factor behind the increase in optical transmission loss, and thermal expansion difference causes the cracking or the like in an optical fiber preform in the process of making the optical fiber preform.
In order to overcome such a problem at the time of optical fiber drawing, it is an object of the present invention to provide a dispersion-shifted optical fiber which has a structure for effectively eliminating the causes of deterioration in characteristics at the making stage thereof and is suitable for WDM transmission.
In order to overcome the above-mentioned problem, the dispersion-shifted optical fiber according to the present invention is a dispersion-shifted optical fiber comprising a core region extending along a predetermined axis, and a cladding region provided on the outer periphery of the core region, in which dopant to be added and the contents thereof are adjusted so as to reduce viscosity difference and thermal expansion difference in the vicinity of each interface between the individual regions.
More specifically, the core region comprises: an inner core in which the concentration distribution of a refractive index reducing dopant such as fluorine (F) or the like is adjusted such that the refractive index is higher in its peripheral portion than in its center portion; and an outer core which is provided on the outer periphery of the inner core and which contains a refractive index increasing dopant such as germanium oxide (GeO2) or the like. In particular, the outer core comprises, at least, an inner portion in which the refractive index increases from a center of the dispersion-shifted optical fiber toward a periphery thereof, and an outer portion which is provided between the inner portion and the cladding region and in which the refractive index decreases from the center of the dispersion-shifted optical fiber toward the periphery thereof. Namely, the contents of the refractive index increasing dopant contained in the outer core decreases toward the inner core and the cladding region, respectively. Also, in the outer portion of the outer core in the dispersion-shifted optical fiber according to the present invention, the ratio of the amounts of change in relative refractive index difference with respect to the radius of the outer portion is not greater than 1.0%/xcexcm.
As noted above, when the viscosity difference between the individual regions is made smaller in the vicinity of the interface between the inner and outer cores or the interface between the outer core and the cladding region, drastic changes of internal stress can be suppressed at the time of fiber drawing. As a result, occurrence of structural irregularity and glass defect are restrained in the vicinity of each interface between the individual regions. Further, by making thermal expansion difference between the individual regions lower, a problem such as generation of cracking or the like in an optical fiber preform can be suppressed on the process of making the optical fiber preform.
Here, the inner portion in the outer core refers to the portion which is in contact with the inner core, whereas the outer portion refers to a portion in contact with the cladding region. As a consequence, each of the inner and outer portions can be constituted by one part or at least two parts having different relative refractive index differences with respect to the reference area of the cladding region. An intermediate portion can also be provided between the inner and outer portions. Different refractive index increasing dopants may be added to the respective portions of the outer core.
Namely, the dispersion-shifted optical fiber according to the present invention can be configured such that a first dopant for increasing the refractive index is contained in at least the inner portion of the outer core, whereas a second dopant for reducing the viscosity of the outer core upon melting is contained in at least the outer portion of the outer core.
In the outer core, each of the inner and outer portions can be constituted by a plurality of parts each containing at least one of the first and second dopants. In this case, a refractive index profile which changes stepwise along the radial direction can be realized in the outer core. Such a structure can also restrain the occurrence of structural irregularity and glass defect in the vicinity of each interface between the individual regions at the time of fiber drawing and suppress sudden thermal expansion in the outer core.
Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, the above-mentioned effective area Aeff is given by the following expression (1):                               A          eff                =                  2          ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    r                                                              )                                                          (        1        )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
On the other hand, the refractive index profile is represented by the relative refractive index difference xcex94ni given by the following expression (2):
xcex94ni=(nixe2x88x92ncd)/ncdxe2x80x83xe2x80x83(2)
where ncd is the average refractive index of the reference area (SiO2) in the cladding region, and ni is the average refractive index in each part i constituting the core region. Hence, the relative refractive index difference xcex94ni is expressed with reference to the average refractive index ncd of the reference area in the cladding region. Also, in the specification, the relative refractive index difference is expressed in terms of percentage, and areas having a negative relative refractive index difference indicate those having a lower refractive index than the reference area.
For effectively restraining occurrence of nonlinear optical phenomena, it is preferred that the difference in the refractive index difference xcex94ni between the maximum value in the outer core and the minimum value in the inner core be 1.0% or more. Also, it is preferred that the maximum refractive index in the outer core is the maximum refractive index in the dispersion-shifted optical fiber, and that the minimum relative refractive index difference in the inner core is the minimum value refractive index difference in the dispersion-shifted optical fiber.
It is preferred that the first dopant to be contained in a desirable portion of the outer core includes at least germanium oxide and that the second dopant includes at least phosphorus. The contents of the second dopant is adjusted such as to decrease from the center of the dispersion-shifted optical fiber toward the periphery thereof.
Also, in the dispersion-shifted optical fiber according to the present invention, the cladding region may have a depressed cladding structure comprising an inner cladding which is provided on the outer periphery of the outer core and which has a predetermined refractive index, and an outer cladding which is provided on the outer periphery of the inner cladding and which has a higher refractive index than the inner cladding. When this cladding structure is employed, the outer cladding acts as the reference area in the cladding region.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.